C++ function method via function pointer

Question

I'm trying to understand how function pointer works can't clarified why I get the following err.

#include <iostream>

using namespace std;
enum COLOR {RED,BLACK,WHITE};

class Car
{public:
 Car (COLOR c): color(c) { cout<<"Car's constructor..."<<endl; CarsNum++;}
 ~Car () {cout<<"Car's destructor..."<<endl; CarsNum--;}
 void GetColor () { cout<<"Color of the car is"<<color<<endl;}
 static int GetCarsNum () {cout<<"Static membet CarsNum="<<CarsNum<<endl; return 0;}
private:
COLOR color;
static int CarsNum;

};

int Car::CarsNum=0;

int main()
{
int (Car::*pfunc) () = NULL;
pfunc=&Car::GetCarsNum ();


 Car *ptr= new Car(RED);
 ptr->GetColor ();
 ptr->GetCarsNum ();
 delete ptr;
 ptr=0;
 Car::GetCarsNum();
    return 0;
}

Err msg:

main.cpp|23|error: lvalue required as unary '&' operand

Problem is with:

  pfunc=&Car::GetCarsNum ();

Any help would be greatly appreciated

Please always indicate the exact line the error message mentions. — Deduplicator
– Deduplicator, Commented May 5, 2014 at 14:12

Some programmer dude · Accepted Answer · 2014-05-05 14:13:47Z

2

With &Car::GetCarsNum () you are calling GetCastNum, and taking the return value to make it a pointer (with the address-of operator &).

To solve this simply drop the parentheses:

pfunc=&Car::GetCarsNum;

answered May 5, 2014 at 14:13

Some programmer dude

411k36 gold badges420 silver badges655 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Some programmer dude Over a year ago

@AlexanderBrevig As the member function is static then yes.

kec Over a year ago

@Deduplicator: The answer does have &, though.

user3604569 Over a year ago

In this case I got another err: |23|error: cannot convert 'int()' to 'int (Car::*)()' in assignment|

Some programmer dude Over a year ago

@user3604569 Then you need to drop the Car:: from the declaration of pfunc. It's because the GetCarsNum is a static member function.

grisha · Accepted Answer · 2014-05-05 14:13:21Z

1

No need for parentness:

pfunc=&Car::GetCarsNum;

and

int (Car::*pfunc) () = NULL;

Oh, UPDATE: you have static method: In this case GetCarsNum() is just a simple function:

int (*pfunc) () = &Car::GetCarsNum;

answered May 5, 2014 at 14:13

grisha

1,3072 gold badges14 silver badges21 bronze badges

4 Comments

coder hacker Over a year ago

Why not mention such small answer in comments

grisha Over a year ago

Because then this question will be without answer:)

kec Over a year ago

I would rephrase to make it clear that not only is there no need for parentheses, the parentheses are actually wrong. (Which is why he was getting the error.)

grisha Over a year ago

@kec, Tejas Patel sorry for my quick, unclear answer

Anon · Accepted Answer · 2014-05-05 14:19:05Z

0

I think that by putting braces behind the method, you call the method. This confuses the compiler. who now thinks the & operator is used as binary operator. So remove the () and only specify the function name.

answered May 5, 2014 at 14:19

Anon

1812 silver badges2 bronze badges

Comments

user3604569 · Accepted Answer · 2014-05-07 10:09:43Z

0

Thanks, guys. Now I see the diffrence between pointers to static method (in this case we use syntaxis as for simple function)

int (*pfunc) () = &Car::GetCarsNum;

int this case both with '&' and without '&' gives the same result.(and what is the difference). and calling it:

pfunc();

And other side - function pointer on standart method:

void (Car::*pfunc2) () = NULL;
pfunc2=&Car::GetColor;

and calling it:

(ptr->*pfunc2)();

answered May 7, 2014 at 10:09

user3604569

31 bronze badge

Collectives™ on Stack Overflow

C++ function method via function pointer

4 Answers 4

4 Comments

4 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

4 Comments

4 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related